1. Field of the Invention
The present invention relates to a thermosensitive recording material, and more particularly to a thermosensitive recording material in which image information formed therein with a thermal printhead can be read even when the recording material having the image is accidentally heated with a heat source and the like.
2. Discussion of the Related Art
Currently, with the diversification of information and needs, various information recording materials have been researched and developed for commercial operation in information recording fields. Under such circumstances, recording materials are used not only for information recording and transmitting, but also as the likes of notes such as lottery tickets. Recording materials for use in the lottery tickets have to have good reliability. Therefore image information in lottery tickets is typically recorded by printing. When a need exists for additional writing of new image information in a lottery ticket on demand, images are recorded by a method such as wire dot printing, laser printing and thermal transfer recording because the images recorded by these printing methods have good reliability such that the images cannot be easily erased.
However, these printing methods have the following drawbacks:
(1) since having complex printing process and mechanism, these printing methods have to use a large printing apparatus; PA1 (2) one or more supplies such as ribbons and toners are needed other than a receiving material, resulting in high running cost; and PA1 (3) it is troublesome to change the supplies such as ribbons and toners. PA1 (1) images can be easily recorded by such a simple method that a recording material is imagewise heated; PA1 (2) a small-sized printing apparatus can be used; and PA1 (3) recording materials are low-priced and easy to handle. Therefore, it is possible to improve the above-mentioned drawbacks of the methods such as wire dot printing, laser printing and thermal transfer recording by using thermal recording materials.
Therefore, persons handling the likes of notes in which images are recorded by one of these methods on demand suffer much inconvenience.
Thermosensitive recording materials, which have been used for recording materials for computers, cash registers, calculators, facsimiles, ticket vending machines, measuring instruments, copiers and label printers, have the following advantages:
However, thermal recording materials have a drawback in that when images formed in a thermosensitive recording material accidentally contacts a heat source, the images become unreadable. This is because the non-colored area of the recording material which surrounds the images achieves a colored state which has substantially the same color and image density as those of the images when heated by the heat source. This trouble also occurs when a heat source closely approaches the image instead of contacting the image.
In attempting to improve the reliability of images formed in thermosensitive recording materials, various methods have been proposed, however, there is no thermosensitive recording material which can produce images which are readable when the images are heated at a relatively high temperature. In detailed description, these methods have proposed thermosensitive recording materials which have a heat resistance such that images formed therein can be read when the recording materials having the images are stored in such a high temperature place as warehouses, automobiles and the like in a summer season. The temperature of warehouses and automobiles in a summer season is from about 60 to about 90.degree. C. However, there is a possibility that the likes of notes such as lottery tickets are accidentally heated at a relatively high temperature of from 100 to 150.degree. C. compared to the case mentioned above, for example, by being accidentally ironed or contacted with boiled water and the like. There is no thermosensitive recording material suitable for this use because it is considered to be difficult to improve thermosensitive recording materials so as to have such high heat resistance.
Thermosensitive recording materials are practically and widely used for lottery tickets in some countries other than Japan because of the advantages mentioned above. The thermosensitive recording materials, which are practically used for lottery tickets in the countries, have poor heat resistance such that when the recording materials are heated at a temperature of from about 100 to about 150.degree. C., the recording materials color black and therefore the previously recorded images become unreadable. In these countries, when a person requests to cash a winning lottery ticket, which colors by being accidentally heated, the conversion into cash is refused due to the reason that he is careless in management of the ticket. In Japan, in such a case, if it is identified that the ticket is a winning ticket, the ticket will be converted to cash. However, in Japan thermosensitive recording materials are not practically used for lottery tickets because of having the drawback mentioned above, whereas the recording materials have the many advantages mentioned above.
Because of these reasons, a need exists for a thermosensitive recording material having good heat resistance such that images formed therein can be read even when accidentally heated to a temperature of from 100 to 150.degree. C.